response to stimuli

a stimulus is a detectable change in the internal/external environment of organism that leads tp a response in the organism

it increases the chance of survival for an organism e.g. can detect and move away from harmful stimuli e.. predators and extremes of temp, or to detect and move towards a source of food The organisms that survive have a better chance of raising offspring and of passing their alleles to the next generation - therefore there is a selection pressure favouring organisms with appropriate responses

receptors which are specific to one type of stimulus

a coordinator formulates a suitable recognise to a stimulus coordination may be at the molecular level or involve a large organ such as the brain

responses are carried out by an effector the response may be at a molecular level or involve the behaviour of a whole organism

they are relatively slow process found in both plants and animals

they also have the more rapid means of communication which is the nervous system which usually have many different receptors and control effectors

each receptor and effectors are linked to a central coordinator of some type the central coordinator connects information from each receptor with the appropriate effector

stimulus - receptors - coordinator - effector - response

it is a simple response whose direction is determined by the direction of the stimulus as a result, a motile organism respond to environmental changes moving its whole body either towards a favourable stimulus or away from an unfavourable one

taxes are classified according to whether the movement is towards the stimulus - positive taxis or away from the stimulus - a negative stimulus and also by the nature of the stimulus

when an organism moves away/ moves towards the light this increases their chances of survival of organisms

when a species will move towards/away from an area with glucose (a nutrient) again this will increase their chances of survival because they use glucose as a source of food

it is a form of response in which the organism does not move towards or away from a stimulus Instead, it changes the speed at which it moves and the rate at which it changes direction If an organism crosses a sharp dividing line between a favourable and an unfavourable environment, its rate or turning increases This increases the chances of a quick return to a favourable environment

its rate or turning may slowly decreases ao that it moves in long straight lines before it turns, often very sharply this type of response tends to bring the organic, into a new region with favourable conditions

woodlice lose water from their bodies in dry conditions When they move from a damp area into a dry one, they move more rapidly and change direction more often This increases their chances of moving back into the damp area. Once back in the damp areas, they slow down and change direction less often. This means they are more likely to stay within the damp area However. if alter some time spent changing direction rapidly they are still in the dry area, their behaviour changes. Instead, they move rapidly in straight lines, which increases their chances of moving through the dry area and into a new damp one In this way, they spend more time in favourable damp conditions than in less favourable drier ones This prevents them from drying out and so increases their chances of survival

it is the growth of a plant in response to a directional stimulus it can be a positive response or a negative response depending on whether it is growing towards (positive) a stimulus or away from a stimulus (negative)

this is when the plants’ shoot grow towards or away from light The response in both cases increases the probability that the roots will gro into the soil, where they are better able to absorb water and mineral ions

plants do not have a nervous system plants respond to -light because light is needed for photosynthesis • gravity- plants need to be firmly anchored in the soil. Roots are sensitive to gravity and grow in the direction of its pull (positive gravitropism) • water- almost all plant roots grow towards water (positive hydrotropism) in order for it to be used in phototropism and other metabolic processes as well as for support

plant growth factors involve hormone-like substances or, more correctly, plant growth factors plant growth factors are produced in small quantities e.g.plant factor indoleacetic acid IAA, which belongs to a group of substances called auxin. IAA controls plant cell elongation

tropism is the directional growth of a plant in response to a directional stimulus In the case of light, we can observe that a young shoot will grow towards light that is directed as it from one side (unilated light). This is known as positive photropism

• Cells in the top of the shoot produce IAA, which is then transported down the shoot • the IAA is initially transported evenly throughout all regions as it begins to move down at the shoot • Light causes the movement of IAA from the light side to the shaded side of the shoot • a greater concentration of IAA builds up on the shaded side of the shoot than on the light side • As IAA causes elongation of the IAA on the shaded side of the shoot, the cells on the side alongside more • The shaded od the shoot elongates faster than the light side, causing the shoot tip to bend towards thelight

IAA causes bending of roots in response to light However whereas a high concentration of IAA increases cell elongation in shoots, it inhibits elongation in roots e.g. an IAA concentration of 10 parts per million increases shoot elongation by 200% but decreases toot elongation by 100% As a result, in roots, the elongation of cells is greater on the light side than on the shaded and so roots bend away from light, that is they are negatively phototropic

• cells in the tip of the root produce IAA, which is then transported along the root 2) the IAA is intially transported to all sides of the root 3) gravity influences the movement of IAA form the upper side to the lower side of the root than in the upper side • a greater concentration of IAA builds up on the lower side of the root than in the upper side • As IAA inhibits the elongation of root cells and there is a greater concentration of IAA on the lower side, the cells on this side elongates less than those in the upper side • The relatively greater elongation of cells on the upper side compared to the lower side causes the root to bend downwards towards the force of gravity